Technical Field
The present invention relates to surface force apparatuses and, more particularly, to surface force apparatuses using a spherical lens and Newton's Rings to measure deflection.
Description of the Related Art
Measuring the force of attraction, or adhesion, between two surfaces is a general problem in materials science. Experimentally, measuring the interaction between planar surfaces is challenging because the surfaces need to be perfectly parallel. To get around this difficulty, many measurement techniques use curved surfaces rather than planar ones. For example, crossed cylinders, or a ball and a flat surface may be used to simplify positioning. If the surfaces are smooth and if the radii of curvature are known, the force of interaction measured using curved surfaces can be related to that of two flat surfaces using, e.g., the Derjaguin Approximation.
The crossed cylinder method is used because precise alignment of the cylinder axes is not needed. The main drawback of this approach is that special samples are needed. The samples must be thin (e.g., 1-2 μm) in order to be sufficiently flexible, and they must be transparent because the separation between the surfaces is measured using optical techniques. In practice, mica is the most widely used substrate. If one is interested in other materials, those materials must be grown or deposited onto the mica surfaces.
Another common technique makes use of a flat surface and a spherical surface. The spherical surface is usually a small silica ball with a radius of only a few microns. The ball is attached to the cantilever of an Atomic Force Microscope (AFM). The AFM is used to measure the force between the ball and a macroscopic, flat surface. The main drawback of this approach is that it is difficult to characterize the chemical and physical state of the ball. For example, it is difficult to determine if the ball is smooth or clean at such small sizes.